1. Field of the Invention
The present invention relates to a solid electrolytic capacitor including an anode formed of a valve metal as well as a dielectric film, a conducting polymer layer, and a cathode extraction layer deposited thereon.
2. Description of Related Art
Solid electrolytic capacitors are used in a manner of being incorporated into power supply circuits of CPU and peripheral circuits thereof in various kinds of mobile information terminals, such as personal computers and mobile phones, various kinds of image information devices, such as digital cameras, and other electronic devices.
In general, in a solid electrolytic capacitor, a dielectric layer is formed on an anode made of a so-called valve metal, such as tantalum, niobium, titanium, or aluminum, and an electrolyte layer and a cathode extraction layer are formed on the dielectric layer. As such electrolyte layer, a metal oxide layer, such as manganese dioxide, or a conducting polymer layer, such as polypyrrole and polythiophene, is used.
In the above-described electronic devices, it is required to lower a resistance component of the solid electrolytic capacitor as low as possible. In recent studies, the ESR (equivalent series resistance) of a solid electrolytic capacitor is further lowered.
Japanese Patent Laid-open Publication No. Hei 7-94368 discloses that graphite powder, conducting polymer compound powder, metal powder, and the like are mixed in a conducting polymer layer so as to cause a surface of the conducting polymer layer to be uneven. When the surface of the conducting polymer layer is formed unevenly, mechanical adhesion between the conducting polymer layer and the cathode extraction layer is improved. Accordingly, the dissipation factor (tangent of loss angle: tan δ), and ESR in a high-frequency region can be reduced.
Japanese Patent Lai-open Publication No. 2002-15956 discloses that in a solid electrolytic capacitor including a first conducting polymer layer and a second conducting polymer layer being laminated, the second conducting polymer layer contains a carbon fiber. The carbon fiber is contained in the conducting polymer layer, so that a specific resistance is reduced and an impedance characteristic and capacitance appearance factor in the high-frequency region is improved.
However, in the solid electrolytic capacitor in which graphite or the like is mixed in the conducting polymer layer and the surface of the conducting polymer layer is formed unevenly, a contact area of an interface between the conducting polymer layer and the cathode extraction layer is increased, but the specific resistance of the conducting polymer layer hardly changes. Thus, ESR cannot be reduced greatly.
In addition, in the solid electrolytic capacitor containing the carbon fiber in the conducting polymer layer, the conducting polymer layer is easily deformed under a high-temperature condition. For this reason, the carbon fiber cannot follow the deformation of the conducting polymer layer. Consequently, there arise problems such as a gap generated between the conducting polymer and the carbon fiber, and damage to the carbon fiber itself. As a result, ESR cannot be sufficiently reduced even if the conducting polymer layer contains the carbon fiber.